This invention relates to partially polymerized resins and processes for preparing them.
Such polymers may be rendered photocurable and may be used in passivation films, photoresists, as insulating layers in fabricating electrical devices, as protective films for semiconductor elements and as interlayer dielectrics in multichip modules and other multilayer electronic circuits.
Johnson et al. disclose in IEEE Transactions On Components, Hybrids, and Manufacturing Technology, Vol. 13, No. 2, June, 1990, that a polymer of ##STR1## 1,3-bis(2-bicyclo4.2.0!octa-1,3,5-trien-3-ylethenyl)-1,1,3,3-tetramethyld i siloxane (hereinafter DVS monomer), now available as a partially polymerized (B-staged) solution in mesitylene from The Dow Chemical Company as CYCLOTENE.RTM. 3022 (hereinafter a partially thermally polymerized DVS monomer or DVS resin) may be used as an interlayer dielectric to fabricate thin film multichip modules. The partially thermally polymerized DVS monomer may be applied by spin-coating a solution of the DVS resin onto a substrate, allowing the solvent to evaporate and then polymerizing by heating to about 250.degree. C. for about five minutes in nitrogen. Catalysts and/or initiators are not required for the polymerization.
U.S. patent application Ser. No. 805,395, filed Dec. 10, 1991; PCT Application No. 92/10,649, filed Dec. 10, 1992 and published as 93/12,055, Jun. 24, 1993; Moyer et al. Proceedings, IEPS, Austin, 1992, p. 37 and Rutter et al. Proceedings, ISHM/IEPS, Denver, April 1992, p. 394 disclose a DVS resin that is rendered photocurable by the addition of at least one photosensitive agent in an amount sufficient to convert the mixture to an organic-insoluble solid upon exposing the mixture to photon radiation.
In a preferred embodiment a neat B-staged DVS resin is mixed with a solvent to precipitate higher molecular weight oligomers which are then used as the photocurable resin. The removal of the monomer, dimer and lower molecular weight oligomers facilitates the photocuring of the resin.
Solvent precipitation of the resin has its drawbacks. A large portion of the DVS resin is rejected by the process. Recycling this portion increases the complexity of the process. Also the solvent used for precipitation differs from the solvent in which the finished resin is dissolved for use. Accordingly, the precipitation solvent must be removed and then the solid resin dissolved again in the use solvent, both for the useful higher molecular weight resin and the recycled low molecular weight resin.
U.S. Pat. No. 4,642,329 discloses methods for partially polymerizing or B-staging benzocyclobutene monomers. Among the ways taught is to solution polymerize the benzocyclobutene in high boiling dipolar aprotic solvents such as amides and sulfones. Lithium salts may be added to solubilize the benzocyclobutene.
This technique has its drawbacks. The partially polymerized monomer must be separated from the high boiling solvent, keeping in mind that distillation would involve temperatures that would further polymerize the benzocyclobutene. Also the use of lithium salts would leave residues of lithium which would be unacceptable if the cured resin were to be used as a dielectric in microelectronics.
It would be desirable to prepare a DVS resin with a low level of low molecular weight oligomers (e.g. monomer, dimer and trimer etc.) in a process which does not yield by-products, does not require the use of additional solvents and can be commercially scaled up.